1. Field of the Invention
This invention relates generally to gas concentrators, and in particular, to a concentrator system designed to allow simple expansion of the rate at which product gas is delivered. The application is particularly directed to portable oxygen concentrators for therapeutic use.
2. Description of the Related Art
The application of oxygen concentrators for therapeutic use is known, and many variants of such devices exist. A particularly useful class of oxygen concentrators is designed to be portable, allowing users to move about and to travel for extended periods of time without the need to carry a supply of stored oxygen. Most of these portable concentrators are based on pressure swing adsorption (PSA) designs which feed compressed air to selective adsorption beds. In a typical oxygen concentrator, the beds selectively adsorb nitrogen, resulting in pressurized, oxygen-rich product gas.
The main elements in an exemplary oxygen concentrator are shown in FIG. 1, which shows that air is drawn in and typically filtered at air inlet before being pressurized by compressor. The pressurized air is directed by a complex valve arrangement through adsorbent beds. An exemplary adsorbent bed implementation is comprised of three columns filled with zeolite powder. The pressurized air is directed through these columns in a series of steps which constitute a PSA cycle. Although many different arrangements of beds are possible as well as a variety of different PSA cycles, the typical result is that nitrogen is removed by the adsorbent material, and the resulting oxygen rich air is routed to a product gas storage. Some of the oxygen rich air is routed back through the bed to flush out the trapped nitrogen to an exhaust. Generally multiple beds, or columns in an exemplary device, are used so at least one bed may be used to make product while at least one other is being flushed, ensuring a continuous flow of product gas.
Such PSA systems are known in the art. It will also be appreciated that the gas flow control through the compressor and the beds in a PSA cycle is complex and requires precise timing and control of parameters such as pressure, temperature and oxygen concentration. Accordingly, most modern concentrators also have a programmable controller, typically a microprocessor, to monitor various parameters. Also present in most portable concentrators is a conserver which acts to ensure that oxygen rich gas is only delivered to a patient when a breath is inhaled, thus using less product than a continuous flow arrangement, which in turn allows for smaller and lighter concentrator designs. A typical concentrator will also include a user/data interface as shown in FIG. 1.
A portable oxygen concentrator should be small, light and quiet in order to be useful, while retaining the capacity to produce a flow of product gas, usually at a flow rate prescribed by a medical practitioner, adequate to provide for a patient's needs. Although fixed site stationary PSA based concentrators have been available for many years, such fixed site units may weigh fifty pounds or more, be several cubic feet in size and produce sound levels greater than 50 dbA. A portable concentrator on the other hand may weigh on the order of 10 lbs, be less than one half cubic foot in size and produce as little as 35-45 dbA sound levels. Yet portable concentrators still need to produce the prescribed flow rate. Thus, portable concentrator designs typically involve a significant amount of miniaturization, leading to smaller, more complex designs compared to stationary units. System size, weight, and complexity constraints for portable concentrators may also limit design choices for mitigating overheating and contamination.
In the design of such a concentrator, system optimization can be critical. Several metrics become important. Primary among these being system specific power, or the electric power input required per unit of product gas output. Weight of the device is another important metric. Characteristics which contribute to these metrics include: run time on a battery of a given size/weight, production of noise; vibration and heat which are strongly correlated to system power consumption; durability and impact requirement; and cost. Secondary considerations extend to efficiencies of PSA cycle operation, compressor function, motor function, and supporting electronic circuitry.
In optimizing the above parameters, oxygen concentrator designers are challenged to make trade-offs. For example, additional weight may allow the device to operate more efficiently. Alternatively, weight reduction may result in higher power consumption. Typically, system optimization requires the designer to provide a value function to each parameter, and then to work to maximize that value function through design trade-offs.
One difficulty that follows this design optimization process is the loss of flexibility. Once a specific design point is selected, the system often constrained. Constituent components, such as valves, tubing, compressor, motor, cooling fans or blowers, and circuit electronics may be selected to meet the needs of this optimized design, but when implemented, can restrict the system to operation over a narrow range. For example, an oxygen concentrator system may be designed to produce between 150 to 750 ml/min of 90% pure oxygen. Elements of the system may be capable of producing a higher top end rate when the entire system is designed with this in mind. For example, compressor/motor assemblies are often selected such that they operate near their limit when produced the specified maximum product flow. If the compressor/motor were over-sized, capable of providing more input gas flow to the PSA cycle, the adsorbent may be able to produce more product gas. However, the design would sacrifice in terms of weight, and would drop in efficiency at the lower flow rates, in exchange for this flexibility. Likewise, valves selected to allow up to a maximum flow rate may yield too much flow restriction when flow exceeds this value. Oversizing the valves may result in weight and power consumption penalties.
Pressure swing adsorption systems, well known in the art, utilize a cyclic pressurization and depressurization of chambers containing selective adsorbent. The efficiency of this system is typically measured in several metrics. The PSA recovery, R, is the product oxygen flow rate, Qp, divided by the intake oxygen flow rate, or
  R  =            Q      p              Q      ⁢                          ⁢      x      ⁢                          ⁢              f                  o          2                    where Q is the input air flow rate, and fO2 is the fraction of oxygen in the intake air, typically about 21%. The Specific Power, P/Qp, described above, is then a function of the air intake flow, Q, the pressure swing ratio PH/PL, the compressor efficiency, hc, the motor/driver efficiency, hm, and the ancillary power PA, as shown in the following equation:
      P          Q      p        =                              ρ          air                ⁢                                  ⁢        x        ⁢                                  ⁢                  C          p                ⁢                                  ⁢        x        ⁢                                  ⁢                  T          inlet                ⁢                                  ⁢                  x          ⁡                      (                                                            (                                                            P                      H                                                              P                      L                                                        )                                                                      (                                          γ                      -                      1                                        )                                    γ                                            -              1                        )                                      R        ⁢                                  ⁢        x        ⁢                                  ⁢                  f                      o            2                          ⁢                                  ⁢        x        ⁢                                  ⁢                  η          c                ⁢                                  ⁢        x        ⁢                                  ⁢                  η          m                      +                  P        A                    Q        p            
In optimization, it may be possible to assign design value to the power and mass of specific solutions, producing an optimization curve relative to one or more design parameters. In general, design of a concentrator system is then completed at or near the optimal design point. Often, design is optimized across a range of operation, such as across the product rate of 0.15-0.75 slpm product delivery.
However, if design is carefully performed such that other systemic restrictions are minimized, other operating ranges may be achievable by the device with either minor modifications or by the addition of ancillary equipment. For example, tubing may be selected to be sufficiently large as to not produce restriction, circuit electronics may be designed to handle higher current loads, and valves may be configured to allow higher inlet and exhaust flows.
One of the objectives of the present invention is to provide design approaches which allow flexibility in operational parameters without requiring complete redesign of the concentrator device.